Techniques to make dinghy foils

I am interested in the techniques used to make high performance dinghy foils. Does anyone have the knowledge or have web references that could explain how it is done and particularly small production runs of foils.

I have noted:

www.philsfoils.com

This site gives a lot of information about one way of making custom foils. I am interested in the problem of production that is a little less labour intensive than Phil's approach appears to be.

Does anyone have information?

 

short production runs

The efficiency of molding is hard to beat, even for fairly short production runs. Yes, it takes time to develop the tooling, but the payback is quite fast.

There are trade-offs to be made between molded blades and those done as a one-off over a male core.

The one-off will _probably_ be lighter for a given strength (no gelcoat, no mat or bog to fill voids between a rough shaped core & the skins, etc). The one-off will have laminates that wrap around the leading edge, where a molded product is typically done as two halves that are then bonded together as a final step (another strength issue).

For a molded product you'll have to engineer it for whatever core you decide on - pour-in foam or foaming epoxy, or shaped CoreCell/Divinycell, or other (typically) lightweight material - you'll _probably_ decide on the skins carrying most of the load. And you'll need a way of addressing point compressive loads such as where a centerboard enters the hull, or the lower fitting on a rudder. These are failure points on both molded and one-off blades, but I've seen more molded blades fail than one-offs.

In between the two approaches there is the possibility of using a sheet good as a core (foam or marine ply), sending a batch out to your local CNC shop to be milled - probably one-sided as double sided milling of foils is tricky unless they're used to it. Then do your hand layup and fair and paint them in batch. Less labour intensive than starting with strip-laminated wood blanks, but there are still strength/weight tradeoffs, and you have to do your engineering homework.

Cheers
Phil

 

The 19' sport boat I am building designed By Anthony Gondola has an interesting way of building it's NACA foils for the rudder and Dagger board. The plans included about 16 profiles or 'stations for the board's cross section. The template includes two centerline 1/2" holes. Instructions are to cut out 2 'slices' of each pattern from 1 1/2" doug fir. These are then stacked together and laminated with epoxy. Threaded rods inserted through the centerline holes provides alignment and clamping pressure. Once laminated it is faired and covered with glass cloth. Specs call for 32 lbs of lead at the lower end of the dagger board to acheive neatral buoancy. Very easy way for an amature like myself building a 1 off to get NACA foils.

 

bjl;

Sounds like an easy method to get the correct shape, but very heavy and inefficient structurally. Due to grain direction and being cut into little slabs the wood is doing no work structurally, and the rods on centerline (neutral axis of the loads) are doing nothing either. All the strength and most of the stiffness of this structure will be from the glass skin.

Major loads on any foil are side loads (bending); loads are highest at the skin, zero on centerline. So the wood grain should run from top to bottom (along the length of the foil) to provide any structural stiffness. You would be much better off to use this method but substitute high-density foam (core-cell) for the fir. Use a couple of wood dowels in place of the threaded rod for alignment.

Once you have the blank built to shape, rout a couple of shallow troughs in the sides from top to bottom at about the quarter cord of the foil. The trough should be about 3-4" wide, 3/16" deep at the top and tapering out to nothing at the tip. Fill these with un-directional carbon tapes. Then cover the entire foil with your glass.

All the best, Tad

 

Phil Locker made a good summary of the advantages and disadvantages of custom male vs moulding in 2 halves and then joining. Are there any other alternatives?

Does anyone use a one piece female mould and do it in one shot?
Are masts or spars ever made in a one piece female mould?
Does anyone know how they make the foils for foiling moths?

Thank you all for your feedback

Brian

 

John Ilett posts here once in awhile. You can find his site at
http://www.fastacraft.com

Cheers
Phil